The present invention relates to the field of automatic transmissions; more particularly, relates to the field of automatic transmissions with torque converters which are equipped with lock up clutches; and, yet more particularly, relates to the field of hydraulic fluid pressure control systems for such lock up clutches.
Various automatic transmissions for automotive vehicles are known in various forms. Such an automatic transmission conventionally includes a gear transmission mechanism which provides a plurality of speed stages and which is set to its various speed stages by selective supply of actuating hydraulic fluid pressures to various hydraulic fluid pressure activated friction engaging mechanisms comprised within the gear transmission mechanism such as multi plate clutches and multi plate brakes, said selective supply of actuating hydraulic fluid pressures being provided from a hydraulic fluid pressure control system, of which many forms are per se well known. Such a hydraulic fluid pressure control system typically requires a supply of line hydraulic fluid pressure for operation, and typically receives this supply of line hydraulic fluid pressure from a hydraulic fluid pressurizing pump by way of a line pressure regulation valve which modifies the output hydraulic fluid pressure produced by the pump by releasing a part of said output pressure back to a hydraulic fluid reservoir through a release port.
Further, such an automatic transmission conventionally includes a fluid torque converter, which provides a fluid coupling between the engine of the vehicle and the gear transmission mechanism, thus eliminating the need for any clutch system for the drive train of the vehicle, and allowing for the vehicle to be stationary while the engine is turning at a low rotational speed at or close to the idling speed without the engine stalling, as well as providing torque multiplication by fluid flow in a per se well known way when the vehicle is being accelerated at relatively low speed and relatively low engine rotational speed. Many such torque converters are of course presently well known. Generally, such a torque converter comprises: a housing of a generally toroidal shape, on the inside of which there are formed a series of vanes which constitute a pump impeller, and fixed to a power input shaft; a pump turbine member mounted within the housing as fixed to a power output shaft; and a stator member mounted within the housing via a one way brake on a fixed member. The housing of such a torque converter is kept filled with hydraulic fluid, which is pumped thereinto and is also drained therefrom as will be more fully explained later, and in a per se well known way the pump impeller, the stator member, and the turbine member cooperate, when the housing of the torque converter is thus filled with hydraulic fluid, to define a toroidal hydraulic fluid flow circulation system, circulation of hydraulic fluid around which in the general circulation fashion of a smoke ring is arranged to transfer torque in a conventional manner between the pump impeller and the turbine member of the torque converter.
This supply of hydraulic fluid for filling the torque converter is typically provided to the inside of the housing thereof via a first channel defined along or beside the central rotational axis thereof--in more detail, via a hole in one of the shafts passing along said central rotational axis or through a space defined between two concentric ones of such shafts; and the draining of hydraulic fluid from the torque converter is also typically performed in a similar manner, through a second such channel. The supply of hydraulic fluid is provided, generally in the art, from a torque converter hydraulic fluid pressure regulation valve which supplies a supply of hydraulic fluid at a regulated torque converter hydraulic fluid pressure, which is generally rather lower than the line hydraulic fluid pressure, to the torque converter.
Further, it has become more and more common nowadays for such a torque converter to be provided with a lock up clutch, which is a mechanical clutch which, when actuated, mechanically couples together the pump impeller and the turbine member of the torque converter with regard to their rotation, so that the above mentioned hydraulic torque transmission between the pump impeller and the turbine member no longer occurs or is relevant. The selective engagement of this lock up clutch is performed by a control device such as a hydraulic fluid pressure control device incorporated in the above mentioned hydraulic fluid pressure control system which controls the engagement of the various gear speed stages of the gear transmission mechanism, according to the operational conditions of the vehicle to which the torque converter incorporating this lock up clutch is fitted. In more detail, generally such a lock up clutch is engaged when the torque converter is required to transmit rotary power at a fairly high rotational speed, at which time the torque conversion function of the torque converter is not substantially required. In such a case, if the lock up clutch is not engaged, then, although the torque converter at this time provides a substantially direct power transmission function between its pump impeller and its turbine member, nevertheless a small amount, such as a few percent, of slippage between the pump impeller and the turbine member will inevitably occur, and this will waste a substantial amount of energy because of the useless churning of hydraulic fluid within the torque converter, and also will cause undesirable heating up of the hydraulic fluid contained within the torque converter. Thus, such a lock up clutch is engaged by the above mentioned hydraulic fluid pressure control device, generally speaking, when and only when the vehicle incorporating the torque converter is being driven at high speed with the gear transmission mechanism in a high gear speed stage, with the internal combustion engine of the vehicle thus operating at fairly high rotational speed, in which circumstances the actual hydraulic torque conversion function of the torque converter is not in fact particularly required. The provision of such a lock up clutch is effective for increasing fuel economy of the vehicle, especially when running on the open road such as on an expressway.
It is well known and conventional for such a lock up clutch to be engaged or disengaged according to the directions of supply and draining of the torque converter hydraulic fluid pressure to and from the interior of the housing of the torque converter. In other words, when the torque converter hydraulic fluid pressure as outlined above is being supplied to one channel which leads to the interior of the torque converter housing, and is being released from another channel, then it is arranged that the lock up clutch is engaged; and when the torque converter hydraulic fluid pressure is being supplied to said other channel, and is being drained from said one channel, then it is arranged that the lock up clutch is disengaged. Thus the supply of torque converter hydraulic fluid pressure to the torque converter from the torque converter pressure regulation valve is used for two purposes: to fill the torque converter with hydraulic fluid; and to selectively engage and disengage the lock up clutch, according to the direction of said supply. And this alternative supply of hydraulic fluid pressure to one channel, and draining from the other channel, and the switching over the channels, is typically performed by using a lock up clutch control valve, of which various forms have been suggested, in the prior art.
A typical prior art such system has controlled the lock up clutch to be engaged when and only when both the highest speed stage of the gear transmission mechanism, such as an overdrive speed stage, has been engaged, and also the vehicle road speed has been greater than a certain predetermined road speed value. This has been done, for example, by controlling such a lock up clutch control valve as described above by a control pressure which has been provided by switching an actuating hydraulic fluid pressure for a one of the friction engaging devices of the gear transmission mechanism which is engaged when and only when said highest speed stage is engaged by a lock up clutch interrupt valve whose valve element has been driven to and fro according to a balance relationship between governor hydraulic fluid pressure (indicative of vehicle road speed) and a spring force.
Such a system is satisfactory in some respects, but in fact it has been realized that it is desirable for the lock up clutch also to be engaged, in some circumstances, when a speed stage of the gear transmission mechanism other than the highest speed stage is engaged. In particular, it has been realized that it would be particularly desirable for the lock up clutch to be engaged, sometimes, when the next to the highest speed stage of the gear transmission mechanism is engaged, from the point of view of improving fuel economy of the vehicle.
However, it would not be acceptable for such an engagement of the lock up clutch to adversely affect the drivability and accelerability of the vehicle, during the engagement of such another speed stage than the highest speed stage. It should be noted that the problem of obtaining good acceleration of the vehicle is not really relevant during vehicle operation in the highest speed stage, since this speed stage is not an acceleration speed stage. However, in the lower speed stages, quick acceleration of the vehicle may be called for, and in this case the torque conversion or torque multiplication function of the torque converter is required, which means that the lock up clutch cannot be engaged at this time. Therefore a new design of lock up clutch control system is required.
Another problem that arises with a lock up clutch control system is that of the failure mode characteristic thereof. In detail, if such a control valve as a lock up clutch control valve of the sort described above should stick in its position in which it engages the lock up clutch, or the so called lock up clutch engage position, and then the vehicle speed should gradually drop until the vehicle comes almost or completely to a halt, then the problem will arise that the engine of the vehicle will snatch, stutter, or stall. This is completely unacceptable, and will render the vehicle inoperable, because the fluid clutch function of the torque converter, which allows the engine of the vehicle to continue rotating at low rotational speed when the vehicle has come to rest, is no longer available. Accordingly, some fail safe system must be provided for ensuring that, even if the lock up clutch control valve should stick, the lock up clutch cannot remain engaged when the vehicle speed is very low. Of course, in the event that, as suggested above, the lock up clutch should be engaged during the engagement of other transmission speed stages than the highest speed stage, then some form of fail safe mechanism must be provided for operation in those speed stages also.